So you are saying using this design a shop vac can pull enough air through .75 mdf to hold down parts? All you really need to do then is put a sheet on top and mill it flat? That is amazing....I would not think that would be possible.
The super glue and tape idea works quite well. As an alternative, try using double sided tape where you would put the glue! No fuss, no muss and it works really good. Holding power is quite high and easier to remove the part without any glue residue to worry about.
I also see that since buildswithbrian called out Gary there has been no response.
Be nice, Gary's information is not only completely correct, he's provided great resources of information on the subject. While low density board is likely superior to MDF, I know several commercial shops that use 3/4" MDF with no holes, they just throw it on and surface both sides to increase the air flow. When it gets to thin, they replace the board the same way. For outhosey who want a YouTube reference, you can watch these two, especially the second one where he is correcting his errors and demonstrates exactly what Gary is talking about.
There are 2 sets of holes in this plan also.....It is not sucking air through solid mdf?? I am confused also
Guys, something to think about.... so a (regular) MDF top that has 1010 1/4" holes AND with the top fly cut (as it will be once surfaced off) is essentially the same as an MDF top with NO holes, skimmed both sides
My (possibly faulty) logic is as follows, so the bottom is not skimmed, however it does have 1010 1/4" dia. gateways for the air to flow from the bottom (vacuum source) up into the interior of the MDF where it can flow sideways making its way toward the top (the only exit path) so in essence, we are pulling vacuum through the open 1/4" holes AS WELL AS through the skimmed surface of the MDF.
keeping in mind of course the entire surface of the table is covered so as not to lose air flow through open holes, which is of course how you are supposed to use the vac. table (air damming open holes)
All this being said, I agree with all concerns about overloading or not cooling the shop vac's motor. I have used the my table for 3 hours non-stop with ALL holes totally covered (no bleeder holes to cool motor) and also have used it leaving a few holes uncovered. totally blocked the motor pitch is different, but it just keeps chugging along, and the vac has not cooked (yet) perhaps my Rigid shop vac is more robust than others... time will tell. It would be great to not have to worry about this, and have a "regenerative" blower type of motor that doesn't need air flow for cooling. I do have an old one I pulled out of a dumpster on a job-site many years ago (works fine) and I have hooked it up to my table as a test, it worked the same as the shop vac. I haven't hooked it up permanently because I will need to fabricated a mounting bracket and hose coupler. The Shop vac. is just simpler to hook up (for now)
I like John's idea of having the motor in a seperate enclosed and sound-proofed area. the shop vac is pretty darn loud
Any thoughts on this ?
I would suspect that the "side bleed" through the open holes would exist with the bottom of the board un-skimmed as the suction draws the air out of the board itself and whatever air lies between the board and the object being held (plus the damming so that the board is entirely covered).
I follow your logic, but you have the explanation backwards. The air flows DOWN through the holes toward the vacuum source on its way to the only exit path (the vacuum). As for the damming...watch the CNCNutz video where he demonstrates that he's providing substantial damming with nothing more than a sheet of newspaper (the paper itself being less permeable than the board). I suspect that the greater suction through the holes would provide less damming with newspaper because the vacuum would be concentrated more where the paper meets the hole, "opening up" the pores in the paper (and maybe even tearing it)
I would posit that, with a totally impermeable sheet of something (like aluminum) covering the entire board, there would be no difference in the amount of holding force of a board that has holes versus one without, provided that both sides of the board have been skimmed. I would likewise posit that, provided the sides of the plenum were sealed, you could gasket the top edge of the sides of the plenum itself and get equal holding force against the impermeable object...up until the point at which it flexes enough to break the seal on its way toward being sucked into the plenum. In all cases where a "perfect seal" is attained, I would think that your motor pitch would be the same once the air is drawn out of the board.
Would it be useful to run a Y off of ones dust collection system to a vacuum table? It could be set up with a blast gate. Maybe not enough volume....just wondering??
Wouldn’t think it would work There is high CFM with a good dust collector but not that much suction I think compared to a shop vac even
So Brian...I for one really appreciate your post and your outstanding effort to enlighten us all about building a vacuum table. Until your post I had never really considered using one, let alone building one. I am amazed at the power that can be generated with something as simple as a shop vac. Not knowing anything about such things I assumed it was something best left to a manufacture who knows all about such things. I am a hobbyist so your plan really got me thinking about building a table. Your video and drawings are superb and generous of you to share. The Axiom machines sort of dictate that the vacuum supply must enter from the side, rather than the bottom, like many designs that I have researched, since your post. I was wondering if the ( i hope my terminology is right) plenum cut into the bottom plate similar to the CNCnut video, with the vacuum attached from the top, would increase the holding power because of the smaller volume of space beneath the spoil board. It seems that could also simplify the design a little. However there is probably some mathematical formulas to calculate the design of the tool path or the configuration of the partitions. I think you are really on to something in your design. I say that because there are a lot of CNC users out there like me that are not into production and could use something like this to hold small projects. A lot of us do not need to hold down large sheets. I would think that a good design like yours that could be made with simple materials and powered with a shop vac could almost be marketed. You have really motivated me to build your table. Right now I only have a lot of "what if's" in my head but you really have me thinking...Thank you.
If there are no "leaks" in the plenum (other than the surface board with or without holes) then hold down force won't be affected by the thickness of the plenum...only the amount of time it takes to evacuate the air from the larger space (which probably couldn't even be measured). If your entire workroom was tightly sealed and the vacuum placed outside of it, you could (theoretically) attain the same level of vacuum within your entire shop (from the same vacuum source) that would exist with the same motor being used in any sized box. The reason that more motors are called for in larger vacuum boxes is due to the inherent leakage (when the damming isn't complete and/or the object itself bleeds air along with leakage where through-cuts are made in the course of cutting out parts) as well as the desire to be able to "zone" the vacuum. I believe that if you covered your entire board with "Visqueen" or something similar and only cut out the area of whatever you're holding down, it would not matter how big the box itself is (length, width, height) for a given vacuum source. Where you'd get into trouble is with a small piece with a vacuum that's too small to provide adequate holding force for the aggregate size of what's being held down.
What IS affected by the thicker box, however, is the thickness of the object you are capable of milling because of the reduced space between the bit at maximum gantry height and the top of your material. Since this box likely won't be used for 3-D carving of thick pieces of wood (clamps would be preferable), this wouldn't pose a problem. If thickness IS a concern, you could always make your box long enough to overhang the "far end" of the table and poke your hose in there from the bottom. Obviously, the "unused part" of the box would be sealed up (just like the sides & bottom) and not drilled or skinned.
OK thanks John....good to know. How critical is it that the air flow through the mdf spoil board which ,would be much more constricted without holes, could cause the vacuum to labor or overheat. Maybe the air pumps mentioned in the other posts can deal with that better. I am also getting that small pieces do not hold as well because of surface area so the extra surface not being used needs to be covered. Is that right?
With or without holes, if the board is completely covered with project and damming, the only flow through the board will be where you've made through-cuts and are actually getting airflow through the vacuum, otherwise the flow is completely occluded in all situations.
Something not previously discussed: If you have holes in the board, then any holes "uncovered" by through cuts will clearly allow air to flow more freely than that which must pass through the board. This is going to lessen the suction on the remainder of whatever is occluding the board (damming and project).
Definitely you need to cover the unused surface of the board to provide adequate suction to the small pieces. Even that may not be enough (depending on what your vacuum is capable of). Think of the suction as "negative psi". The force of the perfect vacuum (which you cannot attain) is the weight of the atmosphere on the other side of the piece (assuming the piece itself is not permeable). This force, in "pounds per square inch" is applied over every square inch of the piece. The smaller the piece, the fewer square inches and the lesser the total force applied to the piece. Think of it this way...if you place a weight of 144 lbs in the form of a 12"x12" piece of steel on a sheet of plywood that's 12" x 12", you have a downward force of 1 psi and the total force required to shift that piece of plywood is 144 lbs. (ignoring the weight of the plywood and disregarding friction for the time being).
If you quadruple the size of the plywood and also the weight, you still have a force of 1 psi on the piece, but you have to move a total of 576# to get it to shift sideways. Your vacuum board (prior to any through cuts of the piece) is going to have the same number of "negative psi" applied to each square inch of the piece, no matter how big it is, but the bigger the piece, the more it will resist sideways movement (which is, of course, what you're mainly interested in). Of course, you also get the benefit of more weight in plywood and more friction in the larger piece.
Post by buildswithbrian on Dec 24, 2018 6:38:34 GMT -5
Posted by Zaxis4.... "So Brian...I for one really appreciate your post < snipped > I would think that a good design like yours that could be made with simple materials and powered with a shop vac could almost be marketed. You have really motivated me to build your table. Right now I only have a lot of "what if's" in my head but you really have me thinking...Thank you." end quote
Zaxis3, thank you I appreciate your kind comments please keep in mind though that my design is predicated from the work of many others, I simply adapted it to work for my situation and machine, and I have posted the video and general instruction as an attempt to help others like yourself in the same situation. A lot of us hobbyists need to improvise and make do with somethings that professionals find less than perfect, sometimes it works out, sometimes it doesn't. but we have to keep moving forward and experimenting, and hopefully sharing our successes (and failures) so we all learn as a group. That is what is so great about the internet and forums like this one. Speaking as someone that grew up (before the internet) I truly appreciate this incredible technology that can bring us all together to create, share and otherwise connect with like-minded people.
This thread has been great... I appreciate all the comments and criticisms (good and bad) that it has generated. looking forward to a new year of experimenting, learning and sharing.
Post by buildswithbrian on Dec 24, 2018 6:48:13 GMT -5
A note to all following this thread... There is a minor correction I need to make on the attached PDF drawing accompanying my table... I built my table with only 2 layers (3/4" MDF) of support blocks but in my PDF drawing I show 3 layers. this was pointed out to me by one of the members of this group that built my table (thank you Mike) this doesn't affect the function of the table in any way, but will lessen the thickness of what you can cut on it (z-axis height) I'm not sure how I did this (brain fart i guess, it was a couple months later I made the drawing)
I will be correcting the drawing when I get time and will re-post it here.
Brian, What would happen if a .25 piece of MDF were placed on top of the spoil with the holes in it? If it still has the suction then it solves the replacement issue also? Just wondering.....
I honestly don't know, haven't tried it... I am fairly sure you would need the ULMDF to even attempt this, and I have not yet acquired any to try it. my guess is (as mentioned in previous posts) is my shop-vac won't have the power to pull enough flow to work, but who knows??
I may give it a try if I can order a sheet thru my commercial supplier when I order a sheet of bamboo plywood (which is in short supply) for another project.
I ran across this about using 2 spoil boards....may be of interest for those seeking information on their vacuum builds
Brian, the way you built the box seems like the only viable way to attach vacuum because of the way the Axioms are set up with the T tracks. I am sure you already thought all of that through though. It would be nice to come in from the bottom or top like most builds out there. Seems like the only way to keep it thin. I have found out that the lDF is scarce and expensive around Denver
Post by jcwoodworking on Jan 4, 2019 17:38:58 GMT -5
Brian's way is not the only way. You can do it with a plenum and bleeder board like Gary suggested (with no holes). That is the route I went. Works fantastic. I used a lighthouse vac motor for the suction though.